1. Field of the Invention
The present invention relates to an encoding and decoding process for a servo positioning system in Hard Disk Drive (HDD).
The invention also relates to an encoder and decoder detector for implementing the above process.
2. Description of the Related Art
As is well known in this specific technical field, in data reading and writing hard disk systems the positioning phase of the reading-writing head is performed with the aid of positioning information that are available on the magnetic support, that is on a magnetic disk sector.
This kind of information, generally called ‘servo’, are written radially on the magnetic disk and, because of their peculiar shape, are denoted ‘servo wedges’.
A possible distribution of the ‘servo wedges’ is shown schematically in the enclosed FIG. 1, not in scale proportion, and in comparison with the other sectors wherein the data are commonly stored in the HDD system.
During the disk rotation the servo fields are periodically read to check both the radial and the angular position of the reading-writing head.
The positioning check and the corresponding scanning algorithm are mainly managed by a controller of the HDD system using a specific software. In this respect, a read and write (RW) channel is provided wherein some devices are implemented for picking-up the angular and radial information for the head positioning.
These angular and radial information are transferred to the control algorithm of the HDD system controller.
Let's briefly see what is the pattern of the ‘servo wedges’ that includes a plurality of fields.
A possible pattern signal is shown in FIG. 2 wherein an initial signal portion is identified as preamble and synchronization field, hereinafter sync field. This initial signal portion is used to assure the synchronization of the sampling system and the correct identification of that portion of the various pattern fields by the RW channel.
The identification of the sync field immediately provides the reference angular signal for the head positioning, computed starting from a synchronous index signal for generating the motor phases controlling the disk rotation.
A subsequent portion of the pattern signal of the ‘servo wedges’, that will be called “gray code”, reports an indent of the disk track. A gray code detector is disclosed for instance in the U.S. Pat. No. 5,920,440.
This invention mainly relates to the servo gray code detection phase.
A final portion of the pattern signal of the ‘servo wedges’ is called ‘burst’ field and allows to define the fine radial positioning of the reading-writing head.
The shape of the burst field is similar to the preamble field but with a wider and controlled amplitude.
The HDD controller is able to determine with high precision the positioning correction of the head by measuring the amplitude of the various burst fields in relationship with the knowledge of the track index.
The RW channel of the HDD system shares a portion of the electronic circuitry used for the data reading and for reading the pattern of the servo wedges.
However, generally the servo fields are not written by the RW channel but fixed in tracks before the HD assembly by suitable writing machines allowing a writing precision higher that that normally available by the RW channel.
Even if different writing techniques are available, the spectral content of the servo wedges is always at the lower frequency if compared to the data frequency, with the servo preamble field tone equal roughly to one half that of the data preamble field.
A larger space given to the servo wedge bit allows of course a better reading.
By sampling at the data frequency it's possible to obtain multiple representations of the written servo wedge bit. This is convenient under the point of view of the RW channel implementation to have a reduced spread between the data working frequency and the servo wedge working frequency.
Keeping in mind the systematic doubling of each information bit, by equalizing the signal a reliable detector may be obtained. Moreover, the input noise is concentrated at low frequencies; so, by sampling twice the same information bit a double signal is obtained but not a double noise.
The RW servo channel equalization is a subcase of the Partial Response scheme normally used in the data sector. FIG. 3 shows a schematic example of the response of the reading system, including the pre-amplifier and the reading head, to an isolated transition. Always in FIG. 3 it is shown the shape of that response after an equalization step to the partial response target ‘1−D2’—with D as the unit delay operator—denoted PR4 and disclosed for instance in the U.S. Pat. No. 6,052,244.
The PR4 partial response equalization scheme seems to be the most suitable for the working channel density during the servo phase, as reported in FIGS. 4 and 5. If we suppose as a basic limit that the minimum distance between two subsequent transactions must be 2Tchannel it's easy to demonstrate that it's impossible to code with rates higher than ½. The simple repetition code reaches the capacity of the limit.
A first prior art solution to provide a better servo gray code detection for the servo positioning system in HDD is provided by a CMOS RW channel chip for HDD systems manufactured by STMicroelectronics and commercially known as “Bramante”. This channel chip supports data rates up to 750 Mbit/s and features advanced signal processing that allows the same channel to be used on a wide range of drives and also with wide tolerance heads and media.
In addition to the data recovery architecture, the channel chip also features a fully synchronous servo detection scheme. A proprietary 4/12 coding scheme for servo gray code paired with a matched Trellis detector can be used to allow better disk formatting compared to other ¼ biphase gray code rates.
The 4/12 coding scheme used by this CMOS RW channel chip is schematically shown in FIG. 6.
As reported in FIG. 7, a trellis PR4 decoding scheme is shown. Some transactions are reported by dotted lines and must be allowed during the preamble phase and on the sync mark.
The 4/12 coding scheme requires a coding and decoding table that is shown in FIG. 8.
According to the teachings of the prior art, other kind of code rates are available on the above RW channel chip. For instance, the well known ¼ gray code rates are automatically supported by the channel chip to overcome any servo formatting legacy issue. A fully embedded modified Discrete Fourier Transform (DFT) burst demodulation scheme provides significant gain in determining head position also reducing the latency of the servo loop.
While the Bramante solution for servo wedges has been developed to exploit the Maximum Likelihood (ML) detection advantage for servo gray field—traditionally detected on a symbol-by-symbol basis—the 4/12 gray scheme does not achieve a significant coding gain. Moreover, the equalization PR target had to compromise with the data detection scheme for implementation complexity issues.